In a crack-up, maybe a write-off (3)
I believe the answer is:
car
'maybe a write-off' is the definition.
I can't tell whether this definition defines the answer.
'in a crack-up' is the wordplay.
'in' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rac' can be found hidden inside 'crack'.
'rac' backwards is 'CAR'.
Can you help me to learn more?
(Other definitions for car that I've seen before include "estate, perhaps" , "One for the road?" , "Auto" , "Motor vehicle" , "Wheeled vehicle" .)